1. Field of the Invention
The invention relates to generation and capture of high contrast, detailed optical fingerprint images, and in particular, to a miniaturized optical sensor combining a trapezoidal right angle prism with a holographic phase grating to produce a fingerprint image without dimensional or spatial distortion. The invention further relates to a method of constructing the holographic phase gratings for the invented miniaturized fingerprint sensor.
1. Description of the Prior Art
Total internal reflection has been used for a long time to optically sense ridges and valleys of a finger surface, i.e., to optically capture a fingerprint. H. J. Caulfield and D. R. Perkins, (Caulfield et al) in U.S. Pat. No. 3,716,301 teaches the use of a prism sensor based on total internal reflection in their holographic finger print recognition system. S. Igaki, S Eguchi, F. Yamagishi, H. Ikeda and T. Inagaki (Igaki et. al.) in a paper published in Applied Optics, Vol. 31, pp. 1794-1802 (1955), disclose a parallel plate sensor using total internal reflection in a flat glass plate to transport a captured fingerprint image in combination with a holographic grating both which retrieve the image reflecting within the plate and which corrects to a degree for dimensional aberration in that image. The device described by Igaki et. al. works in a scattering mode rather than absorption. Accordingly, image contrast is reversed with respect to that obtained by Caulfield et al., and a pair of cylindrical lenses are required for correcting astigmatism in the captured image.
More recently, M. Metz, C. Flatow, Z. Coleman and N. J. Phillips (M. Metz et. al.) have developed an edge lit hologram for capturing a fingerprint images not based on total internal reflection.See Laser Focus World, May, 1994, pp. 159-163 and a paper entitled "The Use Of Edge-lit Holograms For Compact Fingerprint Capture" published in the Conference Proceedings of "Card-Tech Secure-Tech 1995" held Apr. 10-13, 1995, in Washington D.C., pp. 221-228.
The primary disadvantage of existing prism fingerprint sensors is that the fingerprint is compressed in one orthogonal dimension with respect to the other by a factor equal to the cosine of the angle at which the image plane is inclined relative to the normal. To explain, any image in a plane viewed at an angle 45.degree. to the normal is compressed in one dimension by cos. (45.degree.), i.e. by 1/.sqroot.2. Right angle isosceles (45.degree.) prisms are typically described for obtaining fingerprint images.!
Another serious drawback of existing prism fingerprint sensors is that the image plane of the fingerprint emerges from the various sensors inclined with respect to the optical axis. Accordingly, it usually necessary to optically and/or computationally reorient the image to a plane normal (.perp.) to the optical axis for optimal resolution.
The glass plate sensor of S. Igaki et al utilizes a holographic grating to diffract internally reflected light propagating within a glass plate at an angle greater than the critical angle so that the wave can emerge from the plate into the air. S. Igaki et al specifically point out that to create such a grating, one of the two interfering light waves for constructing the hologram must meet the conditions of total internal reflection, that is light cannot enter the holographic recording plate from the air. S. Igaki et al then describe a complicated procedure for creating a suitable holographic grating involving a different wavelength of light. Finally, the glass plate holographic grating sensor described by S. Igaki et al has astigmatism because the spherically divergent waveforms scattered from a fingerprint ridges are diffracted by fringe planes of a holographic grating creating by planar waveforms.
In co-pending U.S. application Ser. No. 08/499,673 filed Jul. 7, 1995 entitled "A PRISM FINGERPRINT SENSOR USING A HOLOGRAPHIC OPTICAL ELEMENT", the Applicant and Thomas M. Corboline describe a fingerprint sensor which includes a holographic phase grating optically coupled with and forming a right angle surface of a prism which diffracts light totally internally reflecting from its external surface to propagate normally (.perp.) back into the prism. In the areas where the ridges of the finger surface are in contact with the grating surface the illuminating light is refracted and absorbed. In the areas corresponding to valleys and pores of the finger surface not in contact with the grating surface, the illuminating light is totally internally reflected. The reflected light is diffracted by the holographic phase grating and emerges from the transmission surface of the prism producing a high contrast, detailed image of the ridges, pores and valleys of the finger surface sensor surface interface oriented in a plane normal to the optical axis. The particular advantage conferred is that the holographic phase grating eliminates image distortion due to dimensional compression. And, because the waveform of the image within and emerging from the invented sensor is planar, there is no necessity for additional optics, other than a good camera lens, to correct image aberrations before recording the image electronically or in film. Nor is there any necessity for optically or computationally correcting or enhancing the recorded images thereafter.
However, the light emerging from the invented sensor with the fingerprint image from the transmission surface of the prism is more or less collimated. For most applications the image must therefore be converged or imaged upon, for example, a 1/3 inch diagonal CCD light sensitive surface using a lens. If the converging/imaging lens is placed close to the transmission surface of the prism, not all the light rays reflected from the grating surface are captured and converged. As a result, the image is brighter in its central region than its periphery causing a hot spot. Such phenomena effectively limits the extent to which the invented sensor with collimated light output can be miniaturized.